1. Field of the Invention
This invention pertains to photopolymerizable compositions, elements, and processes of photopolymerization. More particularly, it pertains to binders for such compositions and processes that yield improved photoresists processable, i.e., developable and strippable, by aqueous alkaline solutions devoid of organic solvents.
2. Description of the Prior Art
Photopolymerizable systems usually comprise monomer, binder and initiator components which are caused to react by imagewise exposure to radiant energy forming polymer with characteristics dependent on the physical properties of the matrix and the degree of polymerization, crosslinking, grafting to binder and consumption of any plasticizers (the monomer itself may perform this role). One method of readout involves physical differences between exposed and unexposed regions of the polymerizable layer distinguished by solubility; this physical property difference is used in a readout method using washoff with various solvents. Compositions of this kind and their use as photosensitive layers or elements are well known in the art.
A particularly useful application for photopolymerizable compositions lies in that area of technology known as photoresists. For example, it is known (e.g., U.S. Pat. No. 3,469,982), to prepare a film resist in the form of a photopolymerizable layer sandwiched between a cover sheet, (e.g., polyethylene film) and a temporary support. A machine is used to laminate the material to copper, copper-laminated board or other materials after the polyethylene film has been stripped off. The material is exposed through a process transparency using a suitable ultraviolet light source. Before or after exposure, the temporary support is stripped and the photopolymer layer subsequently treated with solvent to remove unexposed material. The tough polymerized layer (0.00005 to 0.005 in. or more thick, depending on product type) remaining on the metal, acts as a resist when the metal is treated with an appropriate etchant, for example, ferric chloride for copper. After etching, the photopolymerized material is removed with a stronger solvent at higher temperature, leaving a finished printed circuit.
Among the various methods of readout, dissolving away unexposed unpolymerized areas (i.e., developing) via a solvent is particularly important. Further, in photoresist technology, removal of the polymerized material (i.e., stripping) via a stronger solvent is usually required. Hence, the desirability of having light-sensitive photopolymerizable compositions which can be developed and stripped without the need of conventional organic solvents is manifest. Organic solvents are costly, may be hazardous with respect to toxicity and/or flammability, may become scarce due to petrochemical shortages, and may pollute the air and water.
Now the advantages of such aqueous systems were noted as far back as 1956 in U.S. Pat. No. 2,760,863; however, no specific compositions were described. In spite of these obvious advantages and this 1956 disclosure, as the majority of the known art exemplifies, the developer and/or stripping solvents most frequently taught do contain an organic component, ranging from a small fraction of water-soluble organic compounds up to neat organic liquids, illustrating the fact that satisfactory, solely aqueous processable photopolymerizable compositions devoid of organic solvents, while desirable, are not readily attainable; the majority require an organic component for processing.
Several references do illustrate solely aqueous alkaline processable formulations; e.g., U.S. Pat. Nos. 2,893,868; 2,927,022; U.S. Pat. No. 3,765,898; U.S. Pat. No. 3,887,450; U.S. Pat. No. 3,833,384; and U.S. Pat. No. 3,804,631. However, the formulations described therein, while not without merit in some applications, still exhibit certain deficiencies, especially when used as photoresists. The usefulness of photopolymerizable compositions for dry film resists, which are laminated as a solid film to a substrate, depends on the proper balance of severable properties: aqueous development and stripping but resistance to treating solutions in the polymerized state; tackiness; adhesion to substrate; flexibility; and solubility in noncorrosive, nonflammable coating solvents, among others.
The high degree of tackiness of certain aqueous processable photopolymerizable resist compositions described in the art is a major deficiency. That is, tacky (sticky) photopolymerizable films can be expected to readily accept dirt, including airborne dust, etc., which may yield "pinholes" in the resist. Further, tacky films present obvious potential problems involving lamination alignment during photoresist element preparation. Also, objectionable "pick-off" of tacky, unexposed, unpolymerized areas during delamination, due to adhesion of the sticky areas to the coversheet of a photoresist, may occur.
Soft, low melting compositions are likewise objectionable. On storage in rolls, flow can occur causing edges of the fim to fuse together, lap to lap. If dirt or lint particles inadvertently get wound between the laps at manufacture, the localized pressure spots created cause the resist to flow leading to thin spots in the resist layer which will ultimately produce pinholes in printed circuit manufacture. During lamination, soft films flow excessively at temperatures required to develop adhesion leading to distortions in the laminated coating in the form of ripples of alternating thick and thin areas. Another defect occurs from the heat and pressure during exposure. Edges of the photographic film or masking material emboss the resist, creating thin areas.
Accordingly, the objectives of the instant invention are to provide improved photopolymerizable compositions which remedy the above listed deficiencies of the art compositions. While it may be expected that acidic binders will dissolve in aqueous alkali, it is surprising that including the novel binder combinations of this invention yield photopolymerizable compositions with appropriate adhesion and flexibility for photoresist applications, particularly with improved solely aqueous alkali processability and decreased tackiness and/or softness.
The above objectives, and others which will be apparent to those skilled in the art, are attainable using compositions defined hereinafter.